vanhees71 said:
Well, I think you are right. I shouldn't waste my time anymore to discuss philosophical issues in this forum. It's kind of fighting against religious beliefs rather than having a constructive scientific discussion.
It is even worst. When there is only one philosopher, he throws at you terms such as Platonist and Realist. If you bring in another one, you have it: modern realist; post-modern realist; neo realist; “far-right” realist; etc. And with few more philosophers, the realist spectrum may become the entire real line \mathbb{R}. As long as they keep their “classification” for themselves, I am not bothered because what they say about “reality” is not physics.
I (as a physicist) don’t require that a theory correspond to “reality” because I don’t know what reality is. Stephen Hawking once said: “Reality is not a quality you can test with litmus paper”. A physical theory is nothing but a mathematical structure with predictive power. So, it is meaningless to ask whether it corresponds to “reality”. All that we can ask is that its predictions should be in agreement with the experimental results. QM does this extremely good.
“Roger is worried about Schrodinger’s poor cat. Such a thought experiment would not be politically correct nowadays. Roger is concerned because a density matrix that has | \mbox{cat alive}\rangle and |\mbox{cat dead} \rangle with equal probabilities also has |\mbox{cat alive} \rangle + |\mbox{cat dead}\rangle and |\mbox{cat alive}\rangle – |\mbox{cat dead}\rangle with equal probabilities. So why do we observe either \mbox{cat alive} or \mbox{cat dead}? Why don’t we observe either \mbox{cat alive} + \mbox{cat dead} or \mbox{cat alive} - \mbox{cat dead}? What is it that picks the
alive and
dead axes for our observations rather than
alive +
dead and
alive -
dead. The first point I would make is that one gets this ambiguity in the eigenstates of the density matrix only when the eigenvalues are exactly equal. If the probabilities of being
alive or
dead were slightly different, there would be no ambiguity in the eigenstates. One basis would be distinguished by being eigenvectors of the density matrix. So why does nature choose to make the density matrix diagonal in the
alive/dead basis rather than in the
alive + dead / alive – dead basis? The answer is that the |\mbox{cat alive}\rangle and |\mbox{cat dead}\rangle states differ on a macroscopic level by things like the position of the bullet or the wound on the cat. When you trace out over the things you don’t observe, like the disturbance in the air molecules, the matrix element of any observable between |\mbox{cat alive}\rangle and |\mbox{cat dead}\rangle states average out to zero. This is why one observes the cat either dead or alive and not a linear combination of the two. This is just ordinary quantum mechanics. One doesn’t need a new theory of measurement, and one certainly doesn’t need quantum gravity”
S. Hawking in “
The Nature of Space and Time”.
